Also, there are varying degrees of eczema. You could have had it super mild- maybe just a rash. But some people have it much more extreme, and your experience isn't indicative of this person's. Don't assume everyone is the same as you, cuz it sounds like you have it easy.

Explanation-
This person probably has eczema or a similar inflammatory skin condition. When he gets hit hard, the substance (histamine) that we use in part to heal ourselves is sent to that area. But his body sends too much, and it leads to redness, itching, and swelling. It's like an allergic reaction.
This happens to me when I take hot showers- I get red itchy bumps on my body that eventually fade as I cool down.

I have cystic acne
On my back
Big red angry things under the skin
Anyways, people always smack me on the back really hard
Cos they're douchebags
It just makes me see the world through eyes of pure hatred